With the spread of the Internet, in recent years telecommunication networks have infiltrated into homes and enterprises. In addition, optical access networks for providing services in large quantities at higher speed have developed and passive optical network (PON) systems have widely been adopted as optical access systems.
FIG. 39 illustrates the structure of a PON system. A PON system 6 comprises an optical line terminal (OLT) 6a installed on a station side, optical network units (ONUs) #1 through #n installed on a subscriber side, and a star coupler 6b for optical multiplexing and demultiplexing.
The OLT 6a is connected with the ONUs #1 through #n on a 1-to-n basis by an optical fiber cable F via the star coupler 6b and optical packet communication is performed between the OLT 6a and the ONUs #1 through #n. In FIG. 39, optical packets transmitted in burst mode from the ONUs #1 through #n installed on the subscriber side are multiplexed by the star coupler 6b and are received by the OLT 6a. 
FIG. 40 illustrates the level of signals received by the OLT 6a. A horizontal axis indicates time and a vertical axis indicates the level of an electrical signal after an O/E conversion. The ONUs #1 through #n are installed in different subscribers' houses, so the distance between the OLT 6a and each of the ONUs #1 through #n varies. Accordingly, optical packets received by the OLT 6a differ in level. Each time the OLT 6a receives an optical packet transmitted from each ONU, the OLT 6a sets a threshold level for identifying a code.
FIG. 41 illustrates a signal level kept at a constant value by control. A horizontal axis indicates time and a vertical axis indicates a signal level. The optical burst signals which are depicted in FIG. 40 and which differ in signal level are converted by an optical receiving section included in the OLT 6a so that their signal levels will be constant.
FIG. 42 illustrates the rough structure of the optical receiving section. An optical receiving section 50 comprises a photodiode (PD) 51, a preamplifier 52a, a main amplifier 52b, and a step-up circuit 54. The PD 51 O/E-converts the optical burst signals received to electrical signals. The preamplifier 52a amplifies each electrical signal after the O/E conversion. The main amplifier 52b identifies and amplifies the burst signals which differ in input level among different packets, converts the level of the burst signals to a constant level, and outputs the burst signals. The step-up circuit 54 increases voltage inputted from the outside and supplies increased voltage (bias voltage) to the PD 51.
An avalanche photodiode (APD) is used as the PD 51. An APD is a light receiving element. An APD converts light to an electrical signal (photocurrent) when light is directed in a state (reverse bias state) in which bias voltage is applied in the direction (of from a cathode to an anode) in which it is difficult for an electric current to flow. In addition, an APD has the function of multiplying and outputting the electrical signal after the O/E conversion according to the bias voltage. Compared with a conventional pin (p-intrinsic-n) PD, receiving sensitivity can be improved because of this signal multiplication function.
A receiving circuit for widening a dynamic range by changing a multiplication factor according to the level of an input optical signal is proposed as a conventional art using an APD (see, e.g., Japanese Laid-open Patent Publication No. 11-355218 (Paragraphs [0012]-[0018] and FIG. 1)).
Usually an optical signal is inputted to an APD in a state in which certain bias voltage is applied. By doing so, an O/E conversion is made. As a result, if continuous light is received, stable receiving sensitivity is obtained.
An APD to which certain bias voltage is applied has traditionally been used as a light receiving element in the optical receiving section 50 for receiving optical burst signals which differ in input optical level among different packets. However, if the optical receiving section 50 receives a low optical level signal just after receiving a high optical level signal, receiving sensitivity to the low optical level signal becomes lower than ordinary receiving sensitivity.
This problem will now be described in detail. FIG. 43 illustrates the relationship at different received optical levels between bias voltage applied to the APD (VAPD) and a multiplication factor. A horizontal axis indicates bias voltage (V) applied to the APD and a vertical axis indicates the multiplication factor (also referred to as an M value) of the APD.
As can be seen from graphs depicted in FIG. 43, the multiplication factor falls with an increase in received optical level if the bias voltage is fixed. For example, if the bias voltage is fixed at 57 V, the multiplication factor is about 12.5 at a received optical level of −30 dBm, that is to say, at the time of a low optical level signal being received. However, on the other hand, the multiplication factor is about 3.8 and is low at a received optical level of −6 dBm, that is to say, at the time of a high optical level signal being received. This is a characteristic of the APD. When a high optical level signal is inputted, the multiplication factor is low. When a low optical level signal is inputted, the multiplication factor is high.
FIG. 44 illustrates the waveform of optical burst signals inputted. A horizontal axis indicates time and a vertical axis indicates an optical level. Optical burst signals are inputted to the APD 51 of the optical receiving section 50. It is assumed that a high optical level packet p1 is inputted to the APD 51 and that a low optical level packet p2 is inputted to the APD 51 just after the high optical level packet p1.
FIG. 45 illustrates the relationship between the multiplication factor and the waveform of a response from the APD 51. A waveform H1 indicates the multiplication factor, a horizontal axis indicates time, and a vertical axis indicates the multiplication factor (M value). A waveform H2 indicates the waveform of photocurrent which is outputted from the APD 51 and which is converted to voltage, a horizontal axis indicates time, and a vertical axis indicates a level. In addition, the waveform H2 is the waveform of a response to the above optical packets p1 and p2 obtained at the time of constant bias voltage being applied to the APD 51. The optical packet p1 is converted to a high-level electrical signal p1a and the optical packet p2 is converted to a low-level electrical signal p2a. 
It is assumed that the optical burst signals depicted in FIG. 44 are received. When a high optical level signal is received, the multiplication factor of the APD 51 falls. Therefore, when the low-level electrical signal p2a is received just after receiving of the high-level electrical signal p1a, a delay corresponding to an interval T during which the multiplication factor of the APD 51 returns from a low value to a required value (multiplication factor necessary for obtaining normal receiving sensitivity to the low optical level packet p2) occurs as depicted in FIG. 45. As a result, the amplitude of a leading portion of the low-level electrical signal p2a decreases during the interval T (amplitude reduction interval is 500 ns to 1 μs). Receiving sensitivity deteriorates because of this phenomenon.
The cause of this phenomenon will be as follows. While a high optical level signal is being received, a powerful photocurrent flows through the APD 51. Accordingly, the APD 51 generates heat and the multiplication factor falls. After that, the multiplication factor remains low for a short period of time (during the interval T) until the temperature of the APD 51 falls. With conventional optical receivers, as stated above, the multiplication factor of an APD fluctuates according to fluctuations in input optical power for a certain period of time when optical burst signals are received. This leads to distortion of a waveform. As a result, the quality and reliability of receiving fall.
Therefore, if a receiver for receiving optical burst signals is developed, effective measures should be taken to obtain stable receiving sensitivity even in the case of there being a sharp fluctuation in optical power inputted to an APD.